(1) Field of the Invention
The present invention relates to a system and method for automatically controlling a vehicle speed to a desired cruising speed applicable to vehicles.
(2) Background of the Art
A Japanese Patent Application First Publication (Nonexamined) sho No. 61-129338 published on June 17, 1986 exemplifies an automatic vehicle speed controlling system.
The automatic vehicle speed controlling system disclosed in this Japanese Patent Application Publication includes (a) a vehicle speed sensor for generating a signal in proportion to the vehicle speed, (b) a command switch operable by an operator for generating and outputting a cruise command signal, (c) vehicle speed storage means for storing the vehicle speed at a time when the command switch is operated and the cruise command signal is outputted, (d) a throttle actuator for driving he throttle valve in response to an input command signal, (e) a first controller for outputting an acceleration command signal to the actuator for a predetermined period when the cruise command signal is outputted, (f) a second controller for monitoring a change rate of the vehicle speed with time after a first initialize control by means of the first controller and intermittently outputting the acceleration command signal to the throttle actuator until a rate of deceleration of the vehicle equals to or below a constant value, (g) a third controller for producing an acceleration or deceleration command signal to the throttle actuator in correspondence with a difference between the actual vehicle speed and stored vehicle speed so that the vehicle speed coincides with the stored vehicle speed.
The second controller operates after the above-described first initialize control has been carried out by means of the first initialize controller. The third controller operates after the first and second initialize controls of the first and second controllers.
When a set switch in the command switch is changed from the on state to the off state, the system is activated so that a first initial control signal S.sub.1 is outputted from the first initial controller for the predetermined period of time. If a mounting state of the throttle actuator is even and precise in each vehicle, a vehicle speed control at an initial stage is sufficient to settle the vehicle speed at the stored vehicle speed only by means of the series (first and second) of initialize controls.
However, since the throttle actuators are usually mounted in uneven manners for different vehicles (for example, wires for connecting the actuators to the throttle valves often become too loose for different vehicles), the initial vehicle speed control is ended after the further output of the acceleration command signal (second initialize control signal) S.sub.2 until the deceleration rate of the corresponding vehicle falls below the constant rate in order to eliminate vehicle speed errors due to the mounting errors described above. Thereafter, the vehicle speed is maintained constant, outputting acceleration and deceleration command signals S.sub.3 and S.sub.4 (cruise control signals) for the vehicle acceleration or deceleration according to the difference between the actual vehicle speed V.sub.0 and stored vehicle speed V.sub.1.
The reason for requiring the series or initialize controls is that the deceleration control for the vehicle speed is carried out in the above-described automatic vehicle speed controlling system when the set switch is in the on state. When the vehicle speed is changed from the deceleration state to the cruising state, a predetermined acceleration control for the vehicle speed, i.e., the series of vehicle speed initialize controls is needed.
It is noted that the command switch includes an acceleration switch operable by the operator for accelerating the vehicle at a constant acceleration and a resume switch operable by the operator for reducing the vehicle speed after a brake operation is carried out and returning the vehicle speed to the originally stored vehicle speed, other than the set switch.
However, in the automatic vehicle speed controlling system disclosed in the above-identified Japanese Patent Application Publication, the second initialize control is always carried out if the deceleration value exceeds the predetermined value, even if the difference between the actual vehicle speed and stored vehicle speed is extremely small after the first initial control by means of the first controller is completed.
Although the series of the initialize controls are measures adopted to eliminate unevenness of the mounting state of the individual throttle actuators as described above, a smooth change of the vehicle speed to the cruising speed cannot be made due to the unstable abrupt transfer from the deceleration to the acceleration of the vehicle speed if the second initialize control is carried out by means of the second initialize controller when the difference between the actual vehicle speed and stored vehicle speed is small.